Polyelectrolyte hydrogel-based membranes were pioneered by Chang (see, T. M. S. Chang, Artificial Cells, Charles C. Thomas, Spring field IL, 1967, pp. 524-525) in the artificial cell field. This type of microcapsule is comprised of an interfacial film formed between two polyelectrolytes of opposite charge (polyanion and polycation mixtures). The obtained thin film membrane is on the order of a cell in thickness and forms the outer wall of the microcapsule. The capsule core containing the cells, may be either fluid-filled or an immobilizing hydrogel. Multicomponent blends of synthetic, semi-synthetic and naturally occurring macromolecules have also been evaluated for the preparation of immunoisolation barriers for pancreatic islets (A. Prokop, et al., Adv. Polym. Sci., 1998, 136:53-73). Natural occurring polyanions include several polysaccharides (e.g. alginate, carboxymethyl cellulose, carrageenan, cellulose sulfate, gellan gum, gum arabic, heparin, hyaluronic acid, xanthan, carrageenan, dextran sulfate), while the synthetic polyanions can be made of polyvinylsulfate, polyglutamic acid, polystyrene sulfonate, polyvinylsulfonic acid. Polycations include naturally occurring (e.g. chitosan) and synthetic (e.g. polylysine, polyacrylamide, polyallylamine, polyamide, polyethyleneimine).
The majority of the mentioned capsules were either not sufficiently mechanically stable or suffered from other surface or matrix related deficiencies. These deficiencies include poor morphology, such as capsule sphericity and surface smoothness, which is a result of an osmolar imbalance. Membranes are also often leaky (an internal polymer slowly diffuses out through the capsule wall) or shrink in either PBS or in culture media over a period of a few hours. In order to improve the existing binary capsules several approaches have been considered and tested, like capsule coating, crosslinking, chemical adjustment of charge density, combination of low and high molecular weight polyelectrolytes, adjustment of osmotic pressure, polymer grafting, polymer blending, and processing.
Alginate has been a common material employed for microcapsule fabrication to date. Alginate, a negatively-charged polysaccharide, can either be used alone, or in conjunction with positively-charged polylysine to form alginate-polylysine polyelectrolyte complexes. Alginate itself may be ionically crosslinked by divalent cations such as calcium and barium. Although the encapsulating method based on ionic crosslinking of alginate (a polyanion) with polylysine or polyomithine (polycation) offers relatively mild encapsulating conditions and quite stable membranes, their mechanical properties and long-term stability are poor. Moreover, these polymers when implanted in vivo, are susceptible to cellular overgrowth, which restricts the permeability of the microcapsule to nutrients, metabolites, and transport proteins from the surroundings, leading to starvation and death of encapsulated cells.
Methods for polymerization of macromers using visible or long wavelength ultraviolet light (low UV light, 320 nm or greater) have been described (see, e.g., U.S. Pat. No. 6,911,227). In such methods, polymerization of macromers using visible or long wavelength ultraviolet light is used to encapsulate or coat either directly or indirectly living tissue with polymeric coatings which conform to the surfaces of cells, tissues or carriers under rapid and mild polymerization conditions: polymers are formed from non-toxic pre-polymers (macromers) that are water-soluble or substantially water soluble and too large to diffuse into the cells to be coated. Examples of macromers include highly biocompatible PEG hydrogels, which can be rapidly formed in the presence or absence of oxygen, without use of toxic polymerization initiators, at room or physiological temperatures, and at physiological pH.
Polymerization may be initiated using non-toxic dyes such as methylene blue or eosin Y, which are photopolymerizable with visible or low UV light. The process is non-cytotoxic because little light is absorbed by cells in the absence of the proper chromophore. Cells are largely transparent to this light, as opposed to short wavelength UV radiation, which is strongly absorbed by cellular proteins and nucleic acids and can be cytotoxic. Low levels of irradiation (5-500 mW) are usually enough to induce polymerization in a time period of between milliseconds to a few seconds for most macromers. In order to encapsulate living cells for implantation, the polymerization/gelling conditions must not destroy the living cells, and the resulting polymer-coated cells must be biocompatible.
The aforementioned encapsulation methods present several problems and limitations, including the use of methods and/or materials which are thrombogenic or unstable in vivo, or require polymerization conditions which tend to destroy living cells or biologically active molecules; for example, short wavelength ultraviolet radiation (see, e.g., U.S. Pat. No. 6,780,507). In addition, the use of polyelectrolyte hydrogel membranes has been limited due to their weak mechanical strength and sealing problems.
Improved supramolecular membranes for encapsulating and delivering materials are needed.